1. Field of the Invention
The present invention relates generally to electric motor control and more specifically to control of regeneration energy from an induction motor.
2. Description of the Related Art
In an electrically-driven automobile, an important goal is to make the automobile operate like an automobile driven by a traditional internal combustion engine. In furtherance of that goal, one feature of an internal combustion engine which designers of electrically-driven automobiles attempt to emulate is "engine braking". Engine braking is the tendency of an internal combustion engine to slow the automobile when the driver is not requesting the internal combustion engine to accelerate the automobile. Engine braking is the result of an internal combustion engine and its driveline requiring significant torque to rotate when the engine is not generating torque.
An electric motor does not require nearly as large of a torque to rotate as does an internal combustion engine and its driveline. Therefore, without other means to create an engine braking effect, an electrically-driven automobile will feel like it has much less engine braking than an automobile driven by an internal combustion engine.
One way to increase the feel of engine braking in an electrically-driven automobile is to operate the electric motor in "regeneration mode". In regeneration mode, the motor operates as a generator. Instead of current flowing to the motor from the motor's electrical power source (generally a battery called a "traction battery"), current flows from the motor to the power source. As those versed in the art recognize, a generator exerts a torque load on the mechanism driving the generator. This torque load can provide the feel of engine braking which would otherwise be substantially missing in an electrically-driven automobile.
Using regeneration to create a feel of engine braking creates a by-product, namely the electrical energy generated when the electric motor runs in regeneration mode. Generally, that energy can be put to very good use in charging the traction battery. The energy can then be used when the motor is once again commanded to generate torque to motivate the automobile.
However, a difficulty occurs when the automobile's traction battery is near-fully charged.
Sodium-sulfur batteries, often considered for use in electric vehicles, become damaged if they are overcharged. Further, when a lead-acid battery is used, overcharging can cause the battery to overheat and its electrolyte to boil.
It is therefore apparent that providing regenerative energy to a near-fully charged battery can be disadvantageous.
One way to solve the problem of overcharging the traction battery is to provide an alternative destination for the regeneration energy. One possible alternative destination is a large bank of load resistors connected to accept regeneration energy when the traction battery is near-fully charged. Although this solution is feasible, such a bank of load resistors can add considerable cost to a vehicle. Another possible alternative destination for excess regeneration energy is to turn on electrical loads in the vehicle (e.g., rear window defogger, heated windshield, and the like) when needed to accept excess regeneration energy when the traction battery is near-fully charged. However, engine braking has been shown to require about 5000 watts of mechanical power under some circumstances. There are generally not enough electrical loads on a vehicle which can be turned on to dissipate the large amount of power which would be generated by the motor in this event.
Another way to solve the problem of overcharging the traction battery is to "plug" the electric motor. Plugging a motor refers attempting to drive the motor (which is rotating in one direction) in the other direction. Although an engine braking effect can be obtained, very high slip occurs in the motor when it is plugged. (Slip is defined as the difference between the frequency of rotation of the rotor of the motor and the frequency of rotation of the magnetic field in the motor.) Such high slip induces very high current in the rotor of the motor. The high rotor current can induce enough heat to damage the rotor.
Because of the lack of desirable ways to emulate engine braking without overcharging the traction battery, a means for providing engine braking without overcharging the traction battery will provide advantages over the prior art.